For a mobile communication system to operate satisfactorily, the transmitter and receiver must be locked in both time and frequency. This can be achieved at the receiver by controlling the frequency of the local oscillator used to down-convert the signal received at the antenna from radio frequency (RF) to base-band.
In practice there can exist a frequency error or offset in a signal received by a radio receiver. The frequency offset may be caused by transmitter frequency drift, an instantaneous Doppler frequency shift on the air interface caused by a change in the distance between a mobile terminal and a base station due to the movement of the mobile terminal, or by RF down-conversion frequency error. The presence of a frequency offset can result in inter carrier interference (ICI) and is often detrimental in the estimation process of other parameters, such as the channel gains. Hence accurate frequency offset compensation is important to achieve a satisfactory performance.
The frequency offset is typically estimated in the digital signal processing part of the signal processing chain of the receiver and then used to control the frequency reference produced by the local oscillator via a feedback loop, otherwise known as feedback correction.
This approach is very effective in conditions where the transmitter frequency is stable over time, and there are no Doppler shifts. However in a typical cellular communication system, the handover of the user equipment (UE) between base stations and the mobility of the UE mean that, due to performance degradation, the correction of any frequency offset should be made as quickly as possible.
Alternatively, or in combination with feedback correction, correction of the frequency offset can be performed in the digital signal processing stage, by first estimating the residual frequency offset and then applying a phase correction to the received signal, a feed-forward correction, to immediately remove the residual frequency offset.
The residual frequency offset can be estimated using data-aided frequency offset estimation techniques. Data-aided schemes of frequency offset estimation are based on the transmission of known data blocks, or training sequences, that allow for the estimation of the carrier frequency offset from the estimation of the phase rotation across these blocks at the receiver.
The invention described herein provides techniques for digital data-aided frequency offset estimation aimed at achieving improved accuracy and reduced complexity to provide better performance in the receiver.